Throughout nature, biological processes are found that depend on more than one (simultaneous) protein-interaction. At the present time, it seems that interfering at more than one point in a biological process is going to be more effective than a single interference. This interference may be mediated by two different proteins with binding activity (e.g., antibodies), each binding to an epitope on a target or targets associated with the biological process and subsequently inhibiting the biological process. Particularly, in antibody therapy it is seen that one (monoclonal) antibody is often not effective enough for treating a particular disorder and/or disease. Therefore, the attention of many medical researchers is now focused on combination therapies. Well known examples of combinations of antibodies that are presently clinically pursued are for the treatment of non-Hodgkin's lymphoma, the combination of the already approved anti-CD20 antibody Rituxan with the anti-CD22 antibody Epratuzumab from AmGen, and for the treatment of Hepatitis B, a combination of two human antibodies being developed by XTL Pharmaceuticals (E. Galun et al., Hepatology (2002) 35:673-679). However, the combination of multiple (two or more) drugs (be it binding proteins, antibodies or other) has a number of technical, practical and regulatory drawbacks. In the past, binding proteins (such as antibodies) were typically not designed to function and be produced in combination with one another and development as combinations with optimal clinical efficacy and compatibility may be a problem. As an example, conditions for stabilizing the one may be detrimental to stability of the other(s). Furthermore, multiple sources of recombinant production lead to multiple sources of risks, such as viral contamination, prion contamination and the like.
Historically, the most investigated binding proteins are antibodies. Antibodies normally display binding sites composed of two separate polypeptide chains, assembled as tetrameric protein in the immunoglobulin IgG molecule. More recently, it has become possible to produce single polypeptide chain binding proteins, in which binding is mediated by a single protein domain. Such binding proteins can be based on the same or highly related protein scaffolds or sequence, yet display highly divergent binding specificities. Herein, “SPCBP” is defined as a single polypeptide chain binding protein, and “SPCBPs” as single polypeptide chain binding proteins. Frequently, they are made by first providing a certain level of diversity in a chosen monomeric protein scaffold or fold, which itself can have a natural origin or synthetic basis, and then using molecular selection or screening methods to identify amongst the protein variants those that show a desirable binding specificity. Alternatively, they are harvested from nature, which also has some sources of SPCBPs, such as the “heavy chain only” camelid and shark antibodies.